1. Field of the Invention
The present invention relates to a technique to control positions of lenses.
2. Description of the Related Art
In imaging apparatus such as a video camera and a still image camera, an actuator that operates in synchronization with a drive pulse such as a stepping motor, can be used as a drive source to control driving of a lens such as a variable power lens or a focusing lens at a high positional accuracy. In such an actuator, the lens is driven in an optical axis direction, frequently using a movement mechanism that includes a rotatable feed screw connected to a motor and a rack that engages with the screw.
To downsize the imaging apparatus, it is effective to reduce a drive torque of the stepping motor so as to decrease a power consumption and thereby to miniaturize a power supply battery. In addition, if the imaging apparatus is to be downsized, it is necessary to reduce the drive torque of the motor and to reduce the number of rotations. In the downsized apparatus, a microphone for recording a sound during an image recording and the motor that moves the lenses are closely placed. Accordingly, the drive torque and the number of rotations of the motor need to be reduced to prevent the imaging apparatus from recording a vibration and a noise of the motor. Further, by increasing a pitch of the feeding screw, a lens movement amount per motor rotation amount can be increased. Therefore, the lens can be moved at a high speed while the number of rotations of the motor is reduced.
However, if the torque of the motor is reduced as described above, a step-out phenomenon can occur. In other words, in a case where a drive load is large, even if a drive pulse is applied to the stepping motor, the motor cannot be driven. Therefore, it is difficult to accurately control the position of the lens. On the other hand, if a backlash removal pressure of the rack is decreased to reduce the drive load, a so-called tooth skipping occurs, in which the rack runs off a thread of the screw only with a slight shock. Then, similar to the above case, the position of the lens is not accurately controlled. Further, if the pitch of the feeding screw is increased as described above, a resolution at a stop point of the lens becomes low, and the lens position control accuracy is decreased.
To solve the above problems, it has been proposed to provide a position sensor to detect a position of a lens, for example, in Japanese Patent Laid-Open No. 05-281449.
By providing the position sensor to detect the position of the lens, if the step-out phenomenon or the tooth skipping occurs, the position sensor can detect them and the lens position can be corrected. Further, by increasing a detection resolution of the position sensor high enough, even if a stop resolution becomes lower due to increased pitches of screws, the lens position can be accurately detected.
In such a technique, when the step-out phenomenon or the tooth skipping occurs, the number of drive pulses of the stepping motor that drives the lens can be calculated as follows:the number of pulses=(lens target position−sensor detection position)×pulse conversion coefficient  (1)
Wherein, the pulse conversion coefficient is used to convert a position deviation of a lens detected by a lens position sensor into the number of pulses of a stepping motor. The pulse conversion coefficient is defined as follows:pulse conversion coefficient=(the number of drive pulses per one rotation of motor)×(sensor resolution)/(feed screw pitch)  (2)
In a case where the lens is moved to a target position, including a case where a position is corrected when a lens position deviation occurs due to the step-out phenomenon or the tooth skipping, the stepping motor is driven by the number of drive pulses calculated according to the equations (1) and (2).
However, if the above-described technique is applied to the lens position control, the following problems occur.
First, in the lens movement mechanism, a backlash exists at an engagement part of the feed screw and the rack, or at a coupling part of a lens holding part and the rack. However, in the equations (1) and (2), elements of the backlash are not considered. In an actual movement mechanism, for example, in inverting a drive direction, a mismatch appears between the number of drive pulses of the stepping motor and the movement amount of the lens due to the backlash, and the lens cannot reach the target position. Further, an accumulated inaccuracy due to variations made at the time of manufacturing exists in the pitch of the feed screw. Accordingly, the pulse conversion coefficient calculated according to the equation (2) varies depending on the screws. Because of the screw pitch variations, similar to the above case, there is a mismatch between the number of drive pulses and the movement amount of the lens, so that the lens may not reach the target position.